Simulating \(\rm H_{3}O^{+}\) (aq) and \(\rm OH^{-}\) (aq) ions in water
is more challenging than bulk water.
Although AIMD simulations using different exchange-correlation functionals generally yielded similar
structures and dynamics for solvated hydronium, [1]}, [2]}, [3]}
different GGA functionals such as PW91 [4]}, HCTC [5]}, and BLYP [6]}, [7]}, [8]}, [9]}
gave rise to dramatically different solvation structures and dynamics of solvated hydroxide.
Therefore, the difficulty is particularly prominent in modeling hydroxide
because it has more than one solvation structure.
For example, the hydroxide ion described by PW91 favors a 3-fold coordination structure (tetrahedral structure),
while those described by HCTH and BLYP favor a 4-fold coordination structure (hyper-coordination structure). [8]}
Previous studies proposed that the hyper-coordination structure
strongly suppresses the PT events. [11]}, [8]}
As a result, the two distinct H-bond structures lead to distinct diffusivities
of the hydroxide ion. Although the BLYP functional predicts reasonable diffusion coefficients
of hydroxide, it substantially underestimates the density of liquid water (\(\rm 0.75~g/cm^{3}\) ), [13]}
implying that the BLYP functional is not adequate to describe the sophisticated H-bond network of liquid water.
Importantly, the BLYP functional predicted
that both hydronium and hydroxide ions prefer to experience bursts of PT events
than single PT events within a timescale of water wire compression. [14]}, [15]}
These ubiquitous concerted PT behaviors of the two ions are highly correlated in time, which adds
a new twist to the already sophisticated story but does not provide enough evidence to
explain the slower diffusivity of \(\rm OH^{-}\) (aq) than that of \(\rm H_{3}O^{+}\) (aq).
